Retry only what failed

When a partial batch is retried, feed back the failures and nothing else. Re-running an item that already succeeded repeats its side effects, and for a charge that is a second charge rather than a duplicate row.

The batch runner must know which items are already in. Skipping them is correctness, not an optimisation.

If an item is not safe to run twice, the retry path needs an idempotency key, so a re-run is recognised as the same work. Without the key the retry cannot tell a fresh item from a repeated one, and the only safe choice is to do nothing, which turns a recoverable batch into a stalled one.